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The test fails because it tries to dereference a raw pointer which
points to layer that was destroyed. The fix is to set the pointer
to null and add checks in scrollbar_controller.cc

Bug: 1002828, 987115
Change-Id: I46efc7e35ee62bb36020a42aafbd75998728017c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1854534
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706170}